Optical coherence tomography (OCT) is an optical signal acquisition and processing method. It captures micrometer-resolution three-dimensional images from within optical scattering media. OCT relays on interferometric technique, typically by employing near-infrared light. The use of such relatively long wavelength light allows the light to penetrate into a scattering medium. Depending on the properties of the light source (e.g. where super-luminescent diodes and ultrashort pulsed lasers have been employed), OCT has achieved sub-micrometer resolution (with very wide-spectrum sources emitting over about 100 nm wavelength range).
OCT is one of a class of optical tomographic techniques. A relatively recent implementation of OCT is frequency-domain optical coherence tomography, which provides advantages in signal-to-noise ratio and permits faster signal acquisition. OCT is often used as an optical method in biology and medicine. Commercially available OCT systems are employed in diverse applications, including art conservation, and diagnostic medicine, notably in ophthalmology where it can be used to obtain detailed images from within the retina.
Generally in inspection tasks and particularly in optical metrology there is an ever-increasing importance to the accurate determination of the sensing head distance from the inspected target. This is because of the frequent connection between the system-target distance and the overall measurement accuracy. As OCT operates essentially in the same way as a Linnik interferometer, OCT can be considered for use as a focus sensor like the Linnik interferometer. A Linnik interferometer (as known in the art) is a two-beam interferometer usually used in microscopy, surface contour measurements, topography and optical overlay metrology.
In overlay scatterometry (be it pupil scatterometry or field scatterometry) the overlay mark is commonly a grating-over-grating structure and the overlay information is carried in the relative phase of the lower and upper gratings. In overlay scatterometry of a side-by-side type, the overlay mark (i.e., the metrology target) may comprise a grating next to a grating structure and the overlay information may also be carried in the relative phase of the lower and upper gratings.
In overlay imaging the overlay mark (i.e., the metrology target) can consist separate marks for separate layers and the overlay information is carried in the position of each individual mark on the detector which, in turns, is a result of interferences between different diffraction orders of the individual marks.
Reference is now made to FIGS. 1, 2A and 2B, which schematically demonstrate examples for of a conventional metrology tool 1000 using a Linnik interferometer, as known in the art, which can be used to perform imaging overlay measurements on target structures at the surface of a sample 1060 (e.g. overlay targets at a wafer's surface). The metrology tool 1000, as sown in FIG. 1, can include a light source 1010 optically coupled to illumination optics 1025, potentially with angular and/or spatial dynamic control, for generating a probe beam of radiation 1310. The probe beam 1310 may be turned towards the sample 1060 with a 50/50 beam splitter 1040 (e.g. a Linnik beam splitter). The probe beam 1310 may be focused onto the surface of the sample 1060 with measurement optics 1050 (also noted as measurement objective lens).
Probe beam radiation scattered from the target is collected and collimated by the measurement objective lens 1050 and at least a fraction of the sample beam passes through the beam splitter 1040 and up an optical column of the tool. The fraction of the sample beam may be passed through tube lens/es 1105 and focused on an image detector 1120, such as a Charge-Coupled Device (CCD).
The demonstrated metrology tool 1000 further includes an OCT focusing system having a second beam splitter 1100 (also noted as focus beam splitter), optionally a focus detector lens 1115, a focus detector (FD) 1110 (also noted as focus sensor), reference optics 1080 (also noted as reference objective lens), and a reflector 1090 (e.g. reference mirror surface) that are all arranged in a Linnik-type interferometer configuration. In a Linnik-type interferometer, the reference objective 1080 and the measurement objective 1050 have similar optical performances and complementary optical properties, so that the optical path length of the sample and the reference arms match.
FIGS. 1 and 2A further demonstrate a mechanical shutter 1070, which selectively opens and closes an optical path between the beam splitter 1040 and the reference objective 1080, which focuses a reference beam onto the reflector 1090. The reference beam passes back through the reference objective 1080 toward the beam splitter 1040.
Another fraction of the probe beam power (referred to as the sample beam) is reflected from the sample 1060, and is directed toward the beam splitter 1040, after which it travels together with the reference beam. The two beams are then directed to the focus beam splitter 1100, which directs them both onto the focus detector 1110 where they interfere. Interference fringes are detected at the focus detector 1110 as a result of interference between the sample beam and the reference beam. The interference fringes can be analyzed in order to detect whether the probe beam is in focus at the sample 1060.
The metrology tool as demonstrated in FIG. 2A, demonstrates a device 2000 similar to the device demonstrated in FIG. 1. The metrology tool 2000 may further include a stage (not shown) configured to hold the sample 1060 and a translation mechanism (not shown) may be mechanically coupled to the stage, where the translation mechanism is configured to move the stage in a direction 2061 parallel to the optical axis of the measurement objective lens. FIG. 2B schematically demonstrates an FD's 1110 optional output signal 2200, which can be analyzed in order to detect whether the probe beam is in focus at the sample 1060.
For example, light from sample and reference paths interferes, when lengths of two paths differ by less than coherence length of illumination, accordingly, optimal sample position can be defined as the position where interference contrast is maximal, or at some offset from the position where contrast is maximal.
The common practice for an OCT system can be generally noted as: move, focus, and measure. Usually, the reference mirror 1090 is positioned in a one-time calibration so that maximum interference contrast coincides with maximum contrast of target's image on the imaging sensor 1120; and the following steps are:                Move: at initial step, the sample's stage (not shown) is moved to bring a target structure at the sample's surface into the imaging sensor's field of view.        Focus: at the next step, separation (i.e. distance) between measurement objective 1050 and sample 1060 is scanned in order to modulate fringes formed at focus sensor 1110, as shown in FIG. 2B. Analysis of the FD's output signal 2200 allows the identifying of the separation between measurement objective lens 1050 and the sample 1060, which yields maximum fringe contrast. Accordingly, the separation between the main objective 1050 and the sample 1060 is set to provide a maximal fringe contrast.        Measure: at this step, the sample is imaged by the imaging sensor 1120, where the shutter 1070 of the reference path is closed during acquisition, for the target's overlay measurement.        At the following, the stage is moved to set the measurement illumination onto the next target structure on the sample's surface, which is to be imaged.        
However, the implemented OCT focus system, as discussed above and as illustrated in FIGS. 1 and 2A, has some disadvantages. Until now, no attempt was made to maintain focus during stage moves to each target at the sample's surface. After each stage move, the distance between the measurement objective lens 1050 and the sample 1060 should be scanned to identify position for maximal fringe contrast; sample to objective separation should then be set to such optimal position, after every stage move. Such focusing, after each stage move, increases the time required for each target measurement.
Furthermore, the illumination spectrum is defined once for each target and is the same both for the focusing and the measuring tasks, while the optimal spectrum for the focusing task might be different than the optimal spectrum for overlay sample measurement.
Focusing signals and sample images change with wavelength, for example: a reflectance of different materials on the sample (e.g. an overlay wafer) vary with wavelength; penetration of light to different layers on the sample varies with wavelength; and scatter and diffraction of light from features on the sample vary with wavelength. Focus signals and sample images may change with sample processing variations (e.g. film thickness, resist exposure, etch'depth, and more). The wavelength used for focusing that provides the greatest insensitivity to process and target site variations, may not be the same wavelength that provides the most robust overlay measurements. Accordingly, when a common illumination spectrum is used both for focusing and for measurement tasks, some performance is lost due to the compromised spectrum selection.
Furthermore, because the focusing system uses the same illumination as is used for metrology, the spectrum available for focusing is constrained to the bandwidth used for metrology and does not enable the use of the widest possible spectral bandwidth for focusing. This is disadvantageous because a short coherence length is desired for the OCT-based focusing system and coherence length is inversely related to the bandwidth of the illumination used.
It is within this context that embodiments of the present invention arise.